(a) Field of the Invention
The present invention relates to a compact and high performance lens system for optical recording type disks.
(b) Description of the Prior Art
A lens system to be used for optical recording type disks should have a resolving power about 1.mu. because it should read out the information recorded with high density. Besides, as it is necessary to move the lens system itself for the purpose of automatic focusing and for following up the track on the disk, the lens system should be small in size and light in weight. Moreover, to prevent the lens system from colliding against the disk and causing damage to the disk and/or lens system, the working distance of the lens system should be made long.
To obtain a high resolving power, which is one of the above-mentioned requirements, paraxial aberrations should be corrected favourably and, therefore, spherical aberration and sine condition should be made as small as possible. Besides, it is preferable to make the astigmatic difference small.
To make the lens system small in size and light in weight, the number of lenses constituting the lens system should be made as small as possible. The lens system disclosed in Japanese published unexamined patent application No. 45084/80 is known as an example of lens system for optical recording type disks (video disks) which is small in size and light in weight. However, this lens system has two-component two-element lens configuration as shown in FIG. 1. In case of the above-mentioned lens system, the airspace d.sub.2 between the two lens components is 0.9f&gt;d.sub.2 &gt;0.6f when the focal length of the lens system as a whole is represented by reference symbol f, and the overall length of the lens system is long, i.e., about 1.2f. Moreover, the above-mentioned lens system comprises two lenses and it is not satisfactorily light in weight.